1. Field of the Invention
The present invention relates to a transparent heat shielding material, and in particular relates to a transparent heat shielding multilayer structure with low haze levels.
2. Description of the Related Art
Commercial heat shielding films are primarily silver-based multilayer sputter coatings. Major drawbacks of the heat shielding films include the necessity for expensive sputtering equipment and a multilayer structure for protection of unstable silver coatings. As a result, commercial heat shielding films are often expensive. Accordingly, it would be highly desirable to provide a low cost, highly stable heat shielding film which can be fabricated by an inexpensive, low-temperature coating process.
To avoid the expensive sputtering process, transparent heat shielding films fabricated by simple coating methods have been proposed, in which inorganic metal nanoparticles that absorb particular wavelength bands of radiation are dispersed in a resin binder. However, the inorganic nanoparticles tend to have poor dispersity and problems such as visible light scattering, high haze levels, low visible light transmittance, or low heat shielding efficiency may arise.
U.S. Pat. No. 6,911,254 discloses an infrared (IR) absorbing laminate comprising at least one substrate, a reflective film, and an IR absorbing sheet. The reflective film comprises a metallized polyethylene terephthalate sheet or a multi-layer spectrally selective film. The IR absorbing sheet comprises lanthanum hexaboride nanoparticles, indium tin oxide nanoparticles, antimony tin oxide nanoparticles, or mixtures thereof, dispersed in a thermoplastic polymeric matrix.
U.S. Patent Publication No. 20020090507 discloses an optically active film composite including a layer of resin binder having a thickness of less than 6 microns and a pencil hardness of at least 2H. The optically active film composite also includes nanoparticles of at least one metallic compound absorbing light having a wavelength in the range of 1000-2500 nm, and nanoparticles of a second metallic inorganic compound absorbing light having a wavelength in the range of 700-1100 nm. Characteristics of the composite include shielding against infrared light and anti-scratch capabilities.
U.S. Patent Publication No. 20070048519 discloses a solar control laminate comprising a solar control film comprising inorganic infrared absorbing nanoparticles and a polymeric sheet, wherein the polymeric sheet and the solar control film are adjoining. The inorganic infrared absorbing nanoparticles comprise nanoparticles of antimony tin oxide (ATO), indium tin oxide (ITO), lanthanum hexaboride (LaB6) or mixtures of two or more thereof.
U.S. Patent Publication No. 20070135534 discloses a polymer film comprising lanthanum hexaboride and an epoxy agent. The addition of an epoxy agent provides a stabilization effect to the lanthanum hexaboride, which allows for the production of polymer films that advantageously resist environmental degradation. Thus, the polymer films produced have a significant number of applications, such as applications in display devices, applications as protective coverings, and glazing film applications.
Japanese Patent Publication No. 2005047179 discloses a heat ray shielding polycarbonate resin sheet, wherein at least one side of a polycarbonate resin sheet containing average particle sizes of 100 nm or below of lanthanum boride fine particles, is coated with a transparent resin layer containing an ultraviolet absorber.
The above described patents disclose heat shielding films comprising antimony tin oxide (ATO), lanthanum hexaboride (LaB6), alone or in combination, dispersed in resin binders. However, according to studies, a single layered film comprising ATO suffers from high haze levels and insufficient infrared rejection properties. Additionally, a single layered film comprising LaB6 has poor infrared rejection. Meanwhile, if ATO and LaB6 are combined in the same layer to increase heat shielding performance, haze levels would increase.
Therefore, an improved heat shielding film is needed which can be fabricated by simple coating methods and improve heat shielding performance without increasing the haze levels.